1. Field of the Invention
The present invention relates generally to a fuel control apparatus for an engine and more specifically to a fuel control apparatus for an internal combustion engine which can improve acceleration performance while reducing the quantity of exhausted carbon monoxide.
2. Description of the Prior Art
Various fuel control apparatus for internal combustion engines have so far been proposed. One example of these prior-art apparatus is disclosed in Japanese Patent Kakai (Published and Unexamined) Application No. 56-27040 by the same applicant.
In this prior-art apparatus, first a basic fuel injection quantity T.sub.p is calculated on the basis of an intake air flow quantity Q detected by an air flow meter and an engine revolution speed N detected by an engine speed as T.sub.p =KQ/N (where K is a constant); then various correction coefficients COEF according to engine operating conditions (e.g. coolant temperature), an air-fuel ratio feedback correction coefficient a, and a battery voltage dependent correction value T.sub.s are calculated; and finally a fuel injection quantity T.sub.i at constant engine speed can be calculated as follows: EQU T.sub.i =T.sub.p .times.COEFs.times..alpha.+T.sub.s
Fuel injection quantity calculated as described above is supplied to an engine by applying fuel injection pulse signals (representative of the above calculated fuel injection quantity T.sub.i) to a fuel injection value in synchronism with ignition signals generated for each half turn of an engine crankshaft.
In addition, when acceleration engine operating conditions are detected on the basis of a change in throttle valve opening rate, an initial fuel increment coefficient KAC.sub.0 is determined on the basis of the detected engine operating conditions such as detected coolant temperature detected throttle valve opening rate, etc. In this calculation, the calculated initial fuel increment coefficient KAC.sub.0 is added to the above-mentioned various correction coefficients COEFs in order to increase engine output and therefore improve acceleration performance. Thereafter, the fuel quantity is reduced gradually by decreasing the above-mentioned initial fuel increment coefficient KAC.sub.0 at a constant coefficient decrement rate DKAC in synchronism with the engine operation (e.g. for each half crankshaft revolution).
In the above-mentioned prior-art fuel control apparatus as described above, however, since the initial fuel increment coefficient KAC.sub.0 is fixedly determined when an engine is accelerated and then reduced gradually at a constant coefficient decrement rate DKAC in synchronism with the engine operation, there exist the following problems: when the coefficient decrement rate is determined to a fixed value so that the engine speed rise rate during engine acceleration operation can be optimized, the engine speed can of course be increased at an optimum rise rate as shown by the solid curve in FIG. 1 (the middle). However, even after the engine speed has been increased sufficiently, since the coefficient decrement rate is determined at the same fixed rate, there exists a problem in that an excessive quantity of fuel is supplied to the engine at high engine speed range (overrich in air/fuel ratio) and therefore the quantity of exhausted carbon monoxide CO is excessively increased also as shown in FIG. 1 (the upper).
On the other hand, when the coefficient decrement rate is determined to a large value in order to reduce the quantity of exhausted carbon monoxide, since the fuel quantity to be increased is reduced sharply and therefore the engine speed is excessively reduced as shown by the dashed curve in FIG. 1 (the middle), there exists a problem in that it is impossible to obtain an optimum engine speed rise rate as shown by the solid curve in FIG. 1 (the middle), thus resulting in accel hesitation (when the accel pedal is depressed, the engine cannot be accelerated smoothly) and therefore deterioration in engine acceleration performance.